Weed Management

Weeds are serious yield and profit robbers. Not only do they compete against crop and ornamental plants for moisture and nutrients, but they can also harbor insect and disease pests which then move to your valuable plants.

Use the tabs below to learn basic
weed identification guidelines and how to
effectively control problem weeds you may identify.

What is a weed?

A weed is a plant in an undesirable location. Corn can be a plant in your cornfield but in a bean field it is normally considered a weed. Weeds can rob your desired plants of nutrients, moisture and sunlight, causing lower production and profits. Weeds can also harbor insects and diseases and contaminate grain.

Steps to Avoid Weeds

Grow healthy crops. Weeds grow in spots not occupied by desirable plants. Give your crop or ornamental plants the best possible start by carefully testing and correcting soil fertility, pH, salinity and compaction as needed. Early planting and narrow rows can give your plantings a head start on weeds. Once the desirable plant canopy has filled out, weeds have a much tougher time competing for sunlight, water and nutrients.

Monitor regularly and use weed maps. Always check fields after treating and determine which, if any, weeds have escaped treatment and why.

Identify weeds very carefully. Many herbicides are species specific, and won't have any effect against the wrong weeds.

Learn weed life cycles. When does the seed germinate? When will new seed heads form? When is the weed most susceptible to cultivation or herbicide treatment? The answers to these questions can make a difference between successful weed control and yield loss.

Use action thresholds to make treatment decisions. Do not treat unless your return will more than cover the cost of the treatment. Check with your local Cooperative Extension agent or private consultant for action thresholds for your region.

Know that accurate timing can make the difference between success and failure. Many herbicides have a narrow window of maximum effectiveness. Whichever control method you choose, be sure to take action before weeds go to seed. Always carefully remove seeds from clothing and equipment when moving from an infested area to a clean field.

How to Identify Different Weeds

Weeds can be broken into two main categories: Broadleaves and grasses. Broadleaf weeds are identified by those features that make them different than grasses. For seedlings, this primarily is done by examination of the cotyledon or seed leaf. Cotyledons are the first pair of leaves to emerge. The shape of the cotyledon is a way to initially identify different broad leaf plants. Plants can have linear, oblong, oval, round or butterfly-shaped cotyledons and different extremes of each. The next step in the identification of broadleaves is to notice other features of the plant. Do the true leaves attach on the stems alternately or opposite of each other? How do the leaves attach to the petiole? Are the leaves waxy or hairy, thick or thin? What type of root system does the plant have? A single taproot, tuber, or rhizome can help distinguish between two similar plants.

Grasses can sometimes be a little more difficult to identify. The characteristics that make each different can be very hard to see with the naked eye. Nearly all of the characteristics for grasses are found in the collar region. This is the area of the blade that bends off of the stem. In this area, you need to look for ligules and whether they are absent, smooth, jagged, small or large. Whether or not the area is hairy and how long, and how much and where the hair is found are all important. If a plant has auricles or a triangular-shaped stem, this is a quick way to easily identify the plant.

All of these things need to be noted for correct identification of any weed. Once you know all of the features of the weed species you would like to identify, there are many books, pamphlets and charts to compare your findings. Weeds can vary from region to region, so make sure your reference covers all of the weeds that can be found in your area. Otherwise, you might not find the plant you want to identify listed.

Herbicides are only one of many weapons in your arsenal against weeds. Others are:

Cultivation. Manual or mechanical cultivation is often the best option, especially for post-herbicide spot cleanup. Careful cleanup early in the season is especially important to prevent late-season weed population explosions.

Well-timed cultivation, along with herbicide use, can sometimes allow reduced herbicide use and costs. Shallow cultivation (one to two inches deep) is often best to avoid breaking a previously applied herbicide barrier, bringing up dormant weed seeds, or disrupting desirable plant roots.

Crop rotation. Changing the plants you grow in any one location from season to season can help overall weed management. Weeds that thrive alongside one crop can be suppressed by planting a different crop the following season that is antagonistic to that weed. Crop rotation also aids in herbicide resistance management.

Mowing. Properly timed mowing of weeds, before they form seeds, can be an excellent management tool in pasture and rangeland, row middles, field borders, etc.

Sanitation. Don't allow a thriving weed patch to "seed" your planting areas. Use appropriate controls along ditch banks, fence rows, driveways and other areas that may provide sources of weed seeds. When using straw or hay mulch, be sure it's free of weed seeds.

Flaming. This is useful as a spot treatment or a large scale treatment. Fieldwide burning is still used in some crops, such as asparagus. More commonly, propane-fed burners are mounted on mechanical cultivators or tool bars.

It's not necessary to fully burn weeds to kill them. A sure test is to press your thumb into a flamed weed leaf. If the impression of your thumb remains, weed cell walls have been disrupted and the weed will die.

Flaming also kills insect pests, and does not risk runoff, water contamination, or residue or result in carryover effects on non-target crops.

Biological control. Living plants, animals or even insects can often be used to manage weeds.

Using cover crops and their residues to suppress specific weeds, grazing with livestock, or encouraging or applying insect and disease pests of weeds are techniques that can suppress a number of weed species.

Mulches, barriers and groundcovers. Suppressing weeds by preventing seeds from germinating and growing is the goal. Options include a killed-sod barrier; fabric, plastic or foil mulches; straw mulch; or a cover or inter-planted crop of a non-competing plant, such as sudangrass.

Herbicides. When herbicide application is the best option, be sure to carefully follow label instructions. Consider treating hot spots rather than the entire field. Investigate application technology that "targets" weeds and does not treat bare ground or crop plants. Try alternative herbicides such as soaps, oils or biologicals where appropriate. Note: Always check Cooperative Extension or private consultant recommendations for your crop and region.

How to Scout for Weeds

1.) Make a weed map. Sketch an outline of the location on a piece of paper (or use the sample form provided in this Almanac), noting field boundaries, landmarks, fences, waterways and buildings. Make copies of this map for future use.

2.) Walk the field. Spend some time walking through each quarter of the field, noting on your map the location and relative numbers of each weed type you observe. Be sure to note weeds along field boundaries, drive paths, fences and waterways.

3.) Rank the weeds. Rank them from most to least abundant. Your ranking can be used to determine which weeds are over the action threshold. Select control measures which will eliminate the most important weed pests.

Complete this process in late season when it's still possible to walk through the crop. Use the data you collect to plan weed management actions for next season.

Repeat the process early the next season, and use the information to make last minute adjustments to your plan. Save all of your maps to compare weed problems from year to year and to improve long-term planning.

Using weed maps

Scouting can generate the largest dollar return of any investment you might make in weed management.

Walking fields, orchard blocks or other management units to note the location and abundance of each weed species found is essential in deciding if treatment is necessary and, if so, which control option is best. If you do not scout, you risk throwing herbicide dollars down the drain and sacrificing yields and/or premium quality of your crop or ornamental plants.

Pasture Weed Management

Scouting, accurate identification, knowing life cycles and using thresholds are the keys to weed IPM in pastures and rangeland. Learn which plants can be toxic to your livestock and move them up on the priority list for management.

Mowing can be especially effective in keeping broadleaf and annual weeds below thresholds. Don't move livestock directly from a weedy pasture to a clean pasture, or they are likely to carry weed seeds with them. Be sure any hay brought into pastures is free of weed seeds and noxious plants.

Selective grazing with certain livestock can suppress weed species. For example, goats will feed on gorse, thistle, blackberry and briars; sheep can be effective on dandelion, ragwort and ryegrass; cattle feed on St. Johnswort and sunflower.

Dos and Don'ts of Weed Identification

Do

Don't

Take the time to properly identify weeds.

Forget about other control methods, in addition to chemical controls.

Get a good reference guide to help identify the different species.

Think that plants cannot be identified as seedlings.

Keep records of weed problems to compare from year to year.

Only scout for weeds once in the season, thinking that you will find all of the problems.